when troops of hyænas prowled about the land, together with the other gigantic animals of the southern zone, and tropic clime! Thus it is natural to suppose that in one of the last great revolutions of nature the climate of the earth has undergone a total change; and that originally the now icy north enjoyed a glowing warmth, a rich fertility, and all the fulness of luxuriant life. A number of still more decisive facts declare for this supposition, or, to speak more properly, this certainty; since we discover in the upper parts of Northern Asia, and in general throughout the Polar regions, entire forests of palm in the subterraneous strata, as also well-preserved remains of whole herds of elephants, and of many other kindred species of animals now totally extinct. Long before most of these facts were discovered, Leibnitz had conjectured that originally the earth in general, even in the north, enjoyed a much warmer temperature than in the present period of all-ruling and progressive frost; and Buffon and others have established on this idea their hypothesis of a vast central fire in the interior of the earth. The interior parts of the earth and its internal depths are a region totally impervious to the eye of mortal man, and can least of all be approached by those ordinary paths of hypothesis adopted by naturalists and geologists. The region designed for the existence of man, and of every other creature endowed with organic life, as well as the sphere open to the preception of man's senses, is confined to a limited space between the upper and lower parts of the earth, exceedingly small in proportion to the diameter, or even semi-diameter of the earth, and forming only the exterior surface, or outer skin, of the great body of the earth. Even at a very slight depth below the earth's surface, all change of seasons ceases, and an even temperature eternally prevails, approximating rather to cold, than living heat. Yet on this side the earth is more easy of access than in the upper regions, where not only the higher Alps and glaciers are the last attainable limit to human daring, but even the pure ether of the supernal atmosphere made an aeronaut, celebrated for his disaster, learn at his own cost, how very near is that boundary where, in deadening cold, all life and all observation cease. It is in the physical, as in the moral world--where light and heat should exist, there two things are necessary--a power to give light and communicate heat, and a substance capable of receiving and absorbing the one and the other. Where either condition is wanting, there reigns eternal darkness, and deadly and eternal cold; and so the fact, that the whole action of heat, and of all the life it produces, is confined entirely to this lower atmosphere, should awake attention rather than create surprise. In all matters, even of this sort, we cannot be too mindful of the necessity of confining our researches to that small narrowly circumscribed sphere inhabited by man,